add layout-qualifier to pc_fragColor in fragment shader program for webgl2 (#22931)

Co-authored-by: ligaofeng <[email protected]>

src/renderers/webgl/WebGLProgram.js

@@ -722,7 +722,7 @@ function WebGLProgram( renderer, cacheKey, parameters, bindingStates ) {
 
 		prefixFragment = [
 			'#define varying in',
-			( parameters.glslVersion === GLSL3 ) ? '' : 'out highp vec4 pc_fragColor;',
+			( parameters.glslVersion === GLSL3 ) ? '' : 'layout(location = 0) out highp vec4 pc_fragColor;',
 			( parameters.glslVersion === GLSL3 ) ? '' : '#define gl_FragColor pc_fragColor',
 			'#define gl_FragDepthEXT gl_FragDepth',
 			'#define texture2D texture',